Differences between NAMUR proximity switches and dry contact switches

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1. NAMUR signal: The NAMUR signal is a standard signal for intrinsically safe digital and frequency inputs that is widely used around the world. It was the German standard (DIN19234), and later became the European standard (EN 50227, DIN EN 60947-5-6). The NAMUR signal is a passive two-wire system, powered by 8VDC nominally, with switch signals at 1mA and 3mA. 2. NAMUR-type sensors: All NAMUR-type sensors are intrinsically safe explosion-proof instruments, and they are usually used in conjunction with isolated safety barriers. An isolation barrier supplies 8VDC power to the sensor and detects its current signal. The detection points are typically ≤1.2mA and ≥2.1mA; the isolation barrier converts this switch signal into a relay signal, which is a 0-24V transistor switch signal, either active or passive. NAMUR-type proximity switches are often integrated into valves as valve feedback. 3. NAMUR proximity switch – A proximity switch is a commonly used type of contactless switch. For example, three-wire proximity switches can perform their switching function with a 24V power supply. The NAMUR proximity switch is actually very similar to it; it also requires a power supply of around 8V, but this 8V supply is provided by the isolated safety barrier. After the NAMUR proximity switch is powered on, if no object approaches it, there is a weak current in milliamps in the circuit connected to it. For example, an NAMUR normally open proximity switch, while an isolated safety barrier detects the input signal current. If the detection signal is greater than 2.1 mA, it indicates that an object is approaching the proximity switch; if the detection signal is below 1.2 mA, it means no object is approaching the proximity switch. Then the isolation gate converts it into the corresponding relay signal or transistor switch signal. 4. NAMUR digital signals and dry contact signals: (1) It is easy to distinguish open-circuit faults; for example, with dry contact signals, an open-circuit fault is not easy to detect. For NAMUR switch signals, due to the presence of a weak current signal, it is possible to determine whether there is an open circuit fault in the switch signal by checking the presence or absence of this current signal. Just like in a 2-wire current signal of 4-20mA, since the lower limit of the signal is not zero, it is easy to determine whether the circuit is open. (2) Working principle of NAMUR proximity switches: NAMUR proximity switches are powered by an 8V voltage supplied by a isolation barrier. Since there are open-collector and closed-collector types of these switches, they generate current signals of different magnitudes. For a normally closed NAMUR proximity switch, the current signal is usually between 2.1–6 mA; otherwise, it is between 0.35–1.2 mA. A current below 0.35 mA is generally considered to indicate an open circuit fault in the line, while a current above 6.5 mA indicates a short circuit fault. It’s sufficient to know that the NAMUR standard defines opening and closing based on the magnitude of the current. Due to the very low voltage and current, it is generally used as an intrinsically safe instrument.